Nervous System

Introduction

Tumors of the peripheral nervous system represent approximately 27% of all canine nervous system tumors.¹ These tumors most commonly affect the spinal nerve roots in the caudal cervical and cranial thoracic region and the nerves of the brachial plexus.² A variety of terms has been used to refer to tumors of the peripheral nervous system, including schwannoma, neurilemoma, neurinoma, neurofibroma, and neurofibrosarcoma.² The term Peripheral Nerve Sheath Tumor (PNST) (sometimes also referred to as Malignant PNST) is currently used to refer to these tumors based on their presumptive common cell of origin, the Schwann cell, and similar biologic behavior.² Some pathologists also use the term PNST as synonymous with or closely related to the tumor hemangiopericytoma and place it within the category of soft tissue sarcoma, again based on a presumptive similar cell of origin.³ Hemangiopericytomas are generally found in the skin and subcutaneous tissues–frequently on the limbs-and are characterized by a locally aggressive, but usually systemically passive biological behavior. Although the histiogenesis of PNST involving the spinal nerve roots and plexus nerves and those found in the skin and subcutaneous tissues may be similar, the clinical signs associated with them are very different. The predominant focus of this chapter will be on peripheral nerve sheath tumors which affect the major spinal and cranial peripheral nerves, plexus nerves, and nerve roots.

Peripheral nerve sheath tumors are characterized as being locally aggressive, invasive neoplasms with a very low metastatic potential.² Many sites affected by PNST have been described, including the nerves of the lumbosacral plexus, the sciatic nerve, the thoracic ventral spinal nerve roots, and the trigeminal and vagus nerves.^2,4-8 These tumors are difficult to treat because of their invasive nature and frequent proximity to the spinal cord. One of the most common complications of treatment is tumor recurrence.² Peripheral nerve sheath tumors are uncommon in cats, although there are reports of these tumors causing spinal cord compression at the mid thoracic and thoracolumbar vertebrae.⁹

Histology/Biologic Activity

Peripheral nerve sheath tumors are histologically heterogeneous, comprising cells which are either spindle or oval to round in shape and arranged in interlacing bundles to^10,11 sheets and cords of pleomorphic cells.² Divergent differentiation is seen, with tumors described with fibrous, chondroid, osteoid, myxoid, and squamous and glandular^9,11,12 epithelioid components.² Malignant cellular criteria are typically present,9,10 including anaplasia, multinucleation, high mitotic index, and necrosis.2 Immunohistochemically, most PNST are positive for vimentin and S-100, and negative^10,12,13 for alpha-smooth muscle actin.⁹ The gross characteristics of PNST vary depending on location. Those involving the spinal and plexus nerves often appear as firm, white-grey, fusiform or bulbous thickenings⁹ (Figure 10-1). The masses are typically locally aggressive, extending proximally and distally along the nerve with poor circumscription.^2,7,9,14 The tumor may involve one or multiple nerves within the plexus and can extend through the vertebral foramen into the spinal canal. These tumors do not typically invade the soft tissues surrounding the nerves, but they can invade the spinal cord after extension into the spinal canal.³
 

Clinical Signs

The presenting signs of PNST depend upon the location of the neoplasm and the degree of involvement of the affected nerve tissue. Signs will differ depending on whether the tumor affects a single peripheral nerve, multiple nerves within a plexus, nerve roots, or the spinal cord. Peripheral nerve sheath tumors are usually slow growing, so clinical signs are often present over a period of weeks to months or longer.^2,7,15 Peripheral nerve sheath tumors most commonly affect the nerves of the brachial plexus and the spinal nerve roots in the caudal cervical and cranial thoracic spine.^2,9,14-16 The most common presenting sign of tumors in this location is a chronic, progressive, unilateral forelimb lameness, seen in approximately 78% of cases in one study.² The lameness often has an insidious onset with an unknown cause. The lameness is usually initially weight bearing, but tends to progress to a non weight bearing status over time. Many dogs react painfully to manipulation of the limb and to deep axillary palpation, although the exact painful site is difficult to discern. A palpable mass is present in only approximately 37% of cases.² Moderate to severe muscle atrophy of the affected limb is commonly seen, occurring in approximately 93% of cases in one study.² Paresis and neurological deficits of the affected limb may be seen as the tumor compromises nerve function. Additional signs, including paraparesis, loss of the cutaneous trunci reflex, and ipsilateral Horner’s syndrome can be seen if the tumor extends through the intervertebral foramen to involve the spinal cord. Signs of spinal cord involvement may develop after a period of forelimb lameness, concurrent with the lameness, or as an initial finding depending on the site of origin of the tumor.⁹

Peripheral nerve sheath tumors in other locations manifest with different presenting signs. A smaller population of PNST affects the nerves of the lumbosacral plexus.^2,4 These tumors present with a unilateral hind limb lameness which can progress to unilateral or bilateral hind limb paresis if the tumor invades the spinal canal. Peripheral nerve sheath tumors have been described specifically affecting the sciatic nerve and presented with signs of a hind limb lameness and associated sciatic nerve deficits.⁵ Rectal examination of these dogs revealed a palpable intrapelvic mass not visible on survey radiographs. Peripheral nerve sheath tumors have also been reported to affect the trigeminal nerve.⁷ The main presenting sign of these tumors was unilateral atrophy of the temporalis and masseter muscles, seen in all ten described dogs. One case report described a dog presenting with chronic vomiting, coughing, and signs of respiratory distress and with clinical findings of Horner’s syndrome, ipsilateral laryngeal hemiplegia, and a ventral cervical mass identified via ultrasonography.⁸ At necropsy, a PNST was identified affecting the vagosympathetic trunk. An intrathoracic PNST has been described in a dog which presented for a persistent, productive cough and regurgitation.⁶ This tumor was believed to originate from the ventral thoracic spinal nerve roots.

The differential diagnoses for the most common presenting sign of PNST is any musculoskeletal disorder which produces a forelimb lameness. Many affected dogs have some degree of concurrent elbow or shoulder joint disease which can make definitive diagnosis of the PNST initially difficult. Because there is often a painful reaction on manipulation of the shoulder region, shoulder-area soft tissue injuries, such as biceps tendon or infraspinatus or supraspinatus muscle injuries, may be presumed to be the causative problem.¹⁷ Although chronic musculoskeletal injuries can be associated with muscle atrophy, the atrophy seen with PNST tends to be more severe. Peripheral nerve sheath tumors also must be differentiated from other spinal nerve diseases, such as nerve root disease secondary to intervertebral disc compression.

Diagnostics

Survey radiographs may provide useful information in the workup of PNST. It is useful to characterize orthopaedic disease and to help rule out primary bone diseases such as proximal humeral or vertebral osteosarcoma. The most common described radiographic abnormality with PNST is widening of an intervertebral foramen when tumors extend into the vertebral canal.¹⁸ Survey radiographs are generally of limited use in the diagnosis of PNST because only a small percentage of cases demonstrate detectable abnormalities.

Myelography is a more useful radiographic diagnostic tool and is essential in cases in which there is suspicion of tumor extension to the vertebral canal (Figure 10-2). In one study, approximately 95% of cases with nerve root involvement had abnormal myelograms.² Myelography also accurately identified the lack of macroscopic vertebral canal or nerve root involvement in 9 of 10 cases in which the PNST was located within the brachial plexus. A normal myelogram does not rule out PNST nor does it fully rule out involvement of the nerve roots, but it can be very useful to better plan the surgical approach or approaches needed for treatment.²
 

Advanced imaging techniques including computed tomography (CT) and magnetic resonance imaging (MR) have become valuable tools in the diagnosis of PNST. These imaging modalities have greater diagnostic sensitivity than conventional radiography and can provide important pre-treatment information on tumor localization and the degree of tumor extension.^{4,5,7,14,15,17} Computed tomography was used to identify masses of the brachial plexus in 24 dogs in one study.¹⁵ Twenty of the 24 cases (83%) demonstrated either uniform or heterogenous contrast enhancement. Tumors as small as one cm in diameter were identified; however, it should be noted not all masses identified on CT are associated with neuronal structures. Magnetic resonance imaging has been used in the diagnosis of PNST’s of the radial nerve, trigeminal nerve, and in an intrathoracic location.^6,7,14 The majority of the lesions were isointense on T1-weighted images and either isointense or hyperintense on T2-weighted images (Figure 10-3). All of the lesions demonstrated contrast enhancement. MR is becoming the preferred advanced diagnostic test due to its superior resolution of the tumor boundary and the absence of beam-hardening artifacts.⁷

Electromyography (EMG) is a useful tool in assisting with the diagnosis of PNST. Because of the destructive nature of PNST, the electrical conductivity through affected nerves is frequently altered. A primary goal of EMG, used in conjunction with other diagnostic tests, is to differentiate between muscle atrophy due to denervation and muscle atrophy due to disuse.¹⁸ In the clinical setting, this applies to differentiation between muscle atrophy seen with nerve disease and that seen with orthopaedic disease.^2,7,14 It is not specific for PNST since other types of nerve injury (such as brachial plexus avulsion injuries) can produce EMG changes.19 When used in cases of PNST, EMG can help determine the extent and severity of the nerve damage caused by the tumor, in effect helping to localize the tumor.¹⁹ In one study, all twenty nine dogs in which EMG was performed demonstrated abnormal, spontaneous electrical activity in muscles of the tumor-affected limb.² In a separate study, EMG studies were used to confirm the diagnosis of sciatic nerve tumors in two dogs.⁵ Changes seen on EMG studies include fibrillation potentials, positive sharp waves, and bizarre high frequency discharges.^7,19
 

Surgical Treatment

The goals of treatment of PNST include eradication of the tumor, relief of pain associated with the tumor, and stabilization of neurological dysfunction caused by the tumor. The primary mode of therapy of PNST is aggressive surgical resection of all affected nerve tissue.² The tumor may be approached peripherally if it is located outside of the spinal canal, via a laminectomy if it involves the spinal canal, or from both approaches if the tumor involves both canal and peripheral locations. If the tumor has resulted in severe neurological dysfunction of an affected forelimb, or if resection of the tumor will significantly compromise forelimb function, amputation of the limb may be necessary with resection of the tumor. The basic principle of tumor removal is to resect all affected nerve tissue with a wide margin of grossly normal nerve proximal and distal to the mass. This frequently requires resection of multiple nerve branches due to the highly invasive nature of the tumor. Incomplete excision is common despite aggressive surgical treatment because of the difficulty of discerning normal from abnormal nerve tissue during surgery.² All resected tissue should be submitted for histopathology with proximal and distal edges marked with ink to assist the pathologist in assessing completeness of excision.

Surgical Approaches to the Brachial Plexus

The main and usually best approach to the brachial plexus is the craniolateral approach.²⁰ This provides a wide exposure to the plexus nerves and allows exploration and treatment of the peripheral nerves to the level of the spinal canal. Full exploration of the caudal plexus nerve roots requires transection of the scalenus muscle and cranial rotation of the first rib following an osteotomy near the costochondral junction. The craniomedial approach to the plexus provides better exposure of the peripheral nerves distal to the plexus.^19,20 This approach, though, provides limited access to the proximal portions of the plexus nerves, and it typically involves more muscle dissection than the craniolateral approach.²⁰ Both approaches can be easily modified into a forequarter amputation if the degree of tumor resection will result in a dysfunctional limb.

Craniolateral Approach²⁰

The dog is placed in lateral recumbency with the affected limb, shoulder area, and caudal neck prepared for aseptic surgery. A skin incision is made at the cranial border of the mid scapula and extending distal to the greater tubercle of the humerus. The omotransversarius muscle is transected over the cranial edge of the scapula. Dissection continues ventrally dorsolateral to the cleidobrachialis muscle. The omotransversarius and cleidobrachialis muscles are elevated cranially and cranioventrally, respectively and the scapula is elevated caudally to expose the brachial plexus. The plexus nerves are better defined after separation from the loose subscapular connective tissue. The scalenus muscle may need to be transected to expose the seventh and eighth cervical and first thoracic ventral nerve branches. The first rib can be osteotomized just proximal to the costochondral junction and rotated cranially and laterally to further expose the first and second thoracic ventral nerve roots if these need to be treated as well. This will require ligation of the first intercostal artery and vein and transection of the first intercostal space musculature.

Craniomedial Approach¹⁹

The dog is placed in lateral recumbency, with the affected limb retracted caudally. An incision is made from the caudal aspect of the jugular furrow, medial to the greater tubercle of the humerus, and to the axilla. An incision is made at the medial edge of the cleidobrachialis muscle. The cranial edge of the superficial pectoralis muscle is transected near to its insertion on the humerus. The plexus is exposed by lateral retraction of the limb and blunt dissection around the nerves.

Surgical Approach to the Lumbosacral Plexus

The lumbosacral plexus is a comparatively uncommon site for PNST. In one study, only eight of the 51 cases had tumors affecting either the lumbosacral nerve roots or the sciatic nerve.² Clinical signs associated with tumors affecting the lumbosacral plexus nerve roots include hind limb lameness and hind limb paresis or paraparesis.^2,5 Tumors in this area may be more difficult to locally resect because of the limited access to the lumbosacral nerve trunk. A lumbosacral nerve sheath tumor was completely excised in one study via a hemipelvectomy.⁴

Approach to the Lumbosacral Nerve Trunk²¹

The patient is positioned in ventral recumbency. A dorsal skin incision is made from the craniodorsal iliac spine caudally to the ischiatic spine. The gluteal fascia and underlying superficial gluteal muscle are incised and the sacrospinalis muscle fibers are separated over the dorsal iliac spine and body. The middle gluteal muscle is incised along the dorsal aspect of the ilial wing and body. Blunt intrapelvic dissection following retraction of the middle gluteal and sacrospinalis muscles exposes the lumbosacral nerve trunk.

Laminectomy

A laminectomy is needed in cases in which the PNST extends from a peripheral location into the spinal canal or when the tumor originates at the nerve roots within the canal.^2,12 A hemilaminectomy is usually performed to allow exposure of the nerve roots and the ventrolateral aspect of the spinal cord. The laminectomy may need to be made over several intervertebral spaces if the tumor involves multiple nerves. After exposure of the spinal cord and nerve root, a durotomy is performed to allow transection of the nerve root at the level of the cord. The nerve root is then dissected out from the surrounding epaxial musculature as far as possible. Unless all of the tumor-affected nerve tissue can be removed, a second surgery to remove the diseased tissue from a peripheral approach is necessary. It is more typical, though, that the laminectomy is performed subsequent to a peripheral approach to remove tumor tissue extending into the spinal canal.

Adjuvant Therapy

Chemotherapy and radiation therapy are of questionable benefit in the treatment of PNSTs affecting the major nerves of the brachial and lumbosacral plexes. There is minimal data describing the efficacy of adjuvant therapies for PNST in these locations. The majority of information relative to radiation therapy efficacy refers to the peripherally located, soft tissue sarcoma categorization of nerve sheath tumors (hemangiopericytomas). Radiation therapy as an adjunct to incomplete surgical excision of canine soft tissue sarcomas resulted in a reported disease free interval of 1082 days with a survival rate of 76% at five years.²² If PNSTs affecting the plexus nerves have a biological response similar to those placed in the soft tissue sarcoma category, then adjuvant radiation therapy could be considered an appropriate part of the management of these tumors. The major problem with plexus-located tumors is their proximity to the spinal cord. A recurrent tumor, or a tumor which continues to grow despite radiation therapy will have more profound clinical consequences than those tumors located distally on a limb or on the dog’s trunk, and this will likely lead to shorter disease free intervals and survival times.² At this time, without further data specific to PNSTs affecting the major plexus nerves, radiation therapy can only be considered as a reasonable, but not proved adjunct to surgery.

Prognosis

The prognosis of PNST is generally guarded to poor.² The highly infiltrative nature of PNST and the difficulty of identifying the true extent of the tumor make complete surgical excision difficult to achieve. The proximity of many of these tumors to the spinal canal also limits complete excision. Prognosis has been linked to tumor location. In one study, tumors were divided into three anatomical groups: tumors distal to the brachial or lumbosacral plexus (Peripheral Group), tumors involving nerves within the plexus (Plexus Group), and tumors involving the vertebral canal (Root Group).² The median survival time of dogs in the Root Group was five months. The median survival time of the Plexus group was 12 months. Although there was no statistical difference, the trend was for dogs in the Plexus Group to survive longer than dogs in the Root Group. This survival difference is a reflection of the proximity of the tumor to the spinal cord in the Root Group and the profound clinical effects tumors in this location can have on the patient. Over 82% of all dogs in this study followed to death or at least three months following diagnosis had either recurrence of clinical signs or an unaltered, progressive worsening of presenting clinical signs. Most of the dogs either died directly from or were euthanized due to the effects of the tumor. In the study describing trigeminal nerve sheath tumors, only three of the ten dogs were treated surgically.⁷ One of these cases was alive without disease progression 27 months after surgery, one was alive four months after surgery, and one was euthanized from progressive disease five months after surgery. Survival times of the non-treated cases ranged from five to 21 months.

Conclusion

Peripheral nerve sheath tumors are aggressive tumors which can be difficult to definitively diagnose and successfully treat. Tumor recurrence, or unabated progression of presenting clinical signs are the most common complications of treatment. The hallmark signs of PSNT, which should be an impetus to pursue further diagnostics, are a chronic, progressive forelimb lameness and marked muscle atrophy. The treatment of choice for these tumors is aggressive surgical excision, which may require peripheral excision of the mass, limb amputation, laminectomy, or a combination of these procedures. The efficacy of adjuvant therapies is not clear at this time. The best approach to these tumors will likely be early and aggressive intervention, using diagnostics such as electromyography and MR imaging sooner rather than later in the diagnostic workup, to hopefully identify the tumor before it has had opportunity to invade multiple nerves or the spinal canal. Because of the aggressive nature of these tumors, the overall prognosis of PNST still has to be considered guarded to poor.

References

1. Hayes HM, Priester WA, Pendergrass TW: Occurrence of nervoustissue tumors in cattle, horses, cats and dogs. Int J Cancer 15:39, 1975.
2. Brehm DM, Vite CH, Steinberg HS et al.: A retrospective evaluation of 51 cases of peripheral nerve sheath tumor in the dog. J Am Anim Hosp Assoc 31:349, 1995.
3. MacEwen EG, Powers BE, Macy D, et al.: Soft tissue sarcomas In Withrow SJ, MacEwen EG, eds.: Small animal clinical oncology. Philadelphia: W.B. Saunders Company, 2001, p. 283.
4. Miles JD, Dyce J, Mattoon, JS: Computed tomography for the diagnosis of a lumbosacral nerve sheath tumour and management by hemipelvectomy. J Small Anim Pract 42:248, 2001.
5. Abraham LA, Mitten RW, Beck C et al.: Diagnosis of sciatic nerve tumour in two dogs by electromyography and magnetic resonance imaging. Aust Vet J 81:42, 2003.
6. Essman SC, Hoover JP, Bahr RJ et al.: An intrathoracic malignant peripheral nerve sheath tumor in a dog. Vet Radiol Ultrasound 43:255, 2002.
7. Bagley RS, Wheeler SJ, Klopp L et al.: Clinical features of trigeminal nerve-sheath tumors in 10 dogs. J Am Anim Hosp Assoc 34:19, 1998.
8. Ruppert C, Hartmann K, Fischer A et al.: Cervical neoplasia originating from the vagus nerve in a dog. J Small Anim Pract 41:119, 2000.
9. Braund KG: Neoplasia of the Nervous System In Braund KG, ed.: Clinical Neurology in Small Animals - Localization, Diagnosis and Treatment. Ithaca: IVIS, 2003.
10. Chijiwa I, Ulchida K, Tateyama S.: Immunohistochemistry evaluation of canine peripheral nerve sheath tumors and other soft tissue sarcomas. Vet Pathol 41:307, 2004.
11. Sawamoto O, Yamate J, Kuwamura M et al.: A canine peripheral nerve sheath tumor including peripheral nerve fibers. J Vet Med Sci 61:1335, 1999.
12. Patnaik AK, Zachos TA, Sams AE et al.: Malignant nerve-sheath tumor with divergent and glandular differentiation in a dog: a case report. Vet Pathol 39:406, 2002.
13. Garcia P, Sanchez B, Sanchez MA et al.: Epithielioid malignant peripheral nerve sheath tumour in a dog. J Comp Pathol 131:87, 2004.
14. Platt SR, Graham J, Chrisman CL et al.: Magnetic resonance imaging and ultrasonography in the diagnosis of a malignant peripheral nerve sheath tumor in a dog. Vet Radiology & Ultrasound 40:367, 1999.
15. Rudich SR, Feeney DA, Anderson KL et al.: Computed tomography of masses of the brachial plexus and contributing nerve roots in dogs. Vet Radiology & Ultrasound 45:46, 2004.
16. Carmichael S, Griffiths IR.: Tumours involving the brachial plexus in seven dogs. Veterinary Record 108:435, 1981.
17. McCarthy RJ, Feeney DA, Lipowitz AJ: Preoperative diagnosis of tumors of the brachial plexus by use of computed tomography in three dogs. J Am Vet Med Assoc 202:291, 1993.
18. LeCouteur RA: Tumors of the nervous system In Withrow SJ, MacEwen EG, eds.: Small animal clinical oncology. Philadelphia: W.B. Saunders Company, 2001, p. 500.
19. Farnback CG: Peripheral nerve testing and electromyography In Newton CD, Nunamaker DM, eds.: Textbook of small animal orthopaedics. Philadelphia: J.B. Lippincott Company, 1985, p 1115.
20. Sharp, NJ: Craniolateral approach to the canine brachial plexus. Vet Surg 17:18, 1988.
21. Smith MM, Waldron DR: Approach to the lumbosacral nerve trunk In Smith MM, Waldron DR eds.: Atlas of approaches for general surgery of the dog and cat. Philadelphia: W.B. Saunders Co., 1993, p 350.
22. McKnight JA, Mauldin GN, McEntee MC, et al.: Radiation treatment for incompletely resected soft-tissue sarcomas in dogs. J Am Vet Med Assoc 217:205, 2000.

Peripheral nerve biopsies are routinely performed in veterinary practice and are essential in some cases, along with complete clinical and electrophysiologic examinations, for accurate diagnosis of neurologic disease. General indications for peripheral nerve biopsy include neurologic deficits referable to an anatomic area innervated by the nerve, clinical signs consistent with flaccid paresis or paralysis, hyporeflexia to areflexia, neurogenic muscular atrophy, and sensory deficits of the innervated area. Evaluation of an appropriately collected nerve biopsy may also provide prognostic information, and rarely, a specific etiology for the observed clinical signs.¹

Selection of Biopsy Sites

Although it is possible to biopsy virtually any nerve, whether it be of a mixed, motor, or purely sensory variety, several criteria are used to guide selection of a specific peripheral nerve for biopsy. A priority is that the clinical neurologic examination has provided evidence that the selected nerve is affected by the neuropathy. When possible, clinical evidence of specific nerve involvement is further confirmed by electrophysiologic examination such as nerve conduction studies. Nerves selected for biopsy should be easily identifiable, relatively consistent in their neuroanatomic location, able to be protected from entrapment and recurrent trauma, and accessible with minimal patient morbidity.¹ In addition, ideally the nerve should have published normal, quantitative electrophysiologic and morphometric data available for comparative study, and innervate a skeletal muscle that is amenable to biopsy for which normal data is available.^1-3

In cases where generalized clinical neurologic disease is present, biopsy of the mixed function common peroneal nerve will usually provide a representative specimen. The common peroneal nerve is a preferred biopsy site as many generalized peripheral neuropathies preferentially affect the pelvic limbs prior to the thoracic limbs and normal morphometric and electrophysiologic data for the common peroneal nerve exists for both cats and dogs.^1,3 The common peroneal nerve is also easily visualized as it courses over the lateral head of the gastrocnemius muscle. The flat structure and readily identifiable fascicles make the nerve especially amenable to biopsy. In the pelvic limb, the tibial nerve is a frequently biopsied nerve, as is the ulnar nerve in the thoracic limb. The purely sensory caudal cutaneous antebrachial nerve and caudal cutaneous sural nerve are the most commonly sampled thoracic and pelvic limb nerves in cases in which sensory neuropathy is suspected.

Peripheral Nerve Biopsy Techniques

Peripheral nerve biopsy is usually performed under general anesthesia. There are two basic techniques used to obtain peripheral nerve biopsies, nerve transection and the fascicular biopsy technique. The fascicular biopsy technique is preferred over nerve transection since fascicular biopsy allows for the structural and functional preservation of the majority of the remaining nerve, and thus is associated with minimal or transient clinical morbidity. Specialized equipment is generally not needed, but operating loupes to improve the surgeon’s visualization of the operative field are valuable. The fascicular biopsy technique will be described here through an approach to the common peroneal nerve. Detailed descriptions of the surgical approaches to several other peripheral nerves have been published elsewhere.^1,2

The animal is placed in lateral recumbency, and an area extending from the distal third of the femur to the proximal third of the tibia is prepared for aseptic surgery. The common peroneal nerve can be palpated percutaneously as it courses on the lateral aspect of the stifle just caudal to the proximal tibia and fibula. A 5- to 7-cm oblique skin incision extending from just caudal to the lateral femoral condyle to the proximal fibula will expose the underlying fascia of the biceps femoris muscle, through which the target nerve can be palpated (Figure 10-4). The biceps femoris fascia is elevated and a 5-cm fascial incision made which will allow visualization of the underlying common peroneal nerve as it courses over the lateral head of the gastrocnemius muscle (Figure 10-4- inset). Perineural fat and fascia should be carefully and bluntly dissected off of the visible portion of the nerve. The caudal 1/3 of the proximal end of the exposed nerve is isolated using a ligature of 5-0 or 6-0 silk suture. Gentle traction placed on the proximal ligature allows for the longitudinal division using ophthalmic scissors of a 2- to 4-cm long distal fascicular biopsy specimen. Fascicular biopsy specimens should not exceed 30% of the diameter of the parent nerve from which they are harvested. In the event that the underlying disease process or inherent structure of the nerve complicates visualization of individual nerve fascicles, the exposed nerve segment can be atraumatically spread over a sterile tongue depressor or scalpel handle, which can aid identification of fascicles (Figure 10-5). The biceps femoris fascia is closed with absorbable suture, and the skin closed routinely. Application of an external protective dressing is usually not necessary. Although it was originally reported that neuromas occur frequently following fascicular biopsy, more recent clinical experiences with large numbers of patients suggest that biopsy-related complications are rare.^2,3 In the case of peroneal nerve biopsy, the most commonly reported complication consists of transient proprioceptive deficits and knuckling of the pes, both of which usually resolve within 5 days of the procedure.
 

Processing of Nerve Biopsy Specimens

Nerve biopsies require special handling to avoid artifact formation while in fixative. In order to prevent significant contracture of the biopsy sample, several techniques have been described to maintain the length of the nerve biopsy prior to fixation. These techniques include pinning the nerve at both ends to a section of tongue depressor with 25 to 27 gauge hypodermic needles or securing the nerve to a length of the stem of a standard wooden cotton tipped applicator by placing a circumferential suture of 5-0 or 6-0 silk at either end of the biopsy. The nerve may also be suspended directly in the fixative using a stainless steel weight attached to the free end of the original silk suture that was placed in the proximal portion of the nerve during the biopsy procedure.

Ideally, the specialized laboratory that will be receiving and processing the nerve sample should be contacted prior to performance of the biopsy so that laboratory requests for specific fixatives can be followed. Nerve biopsy specimens are preferably fixed in both 2.5% glutaraldehyde and 10% formalin.¹ If biochemical or specific immunohistochemical studies are desired, snap-freezing of unfixed nerve tissue may be required. Formalin-fixed specimens are embedded in plastic and routinely stained with hematoxylin and eosin, Luxol fast blue, or Gomori trichome stains and evaluated with light microscopy for evidence of axonal degeneration, overt demyelination, or inflammatory or neoplastic cellular infiltrates. Fixation of samples in glutaraldehyde allows for preparation of semithin and ultrathin sections for more detailed light microscopic and ultrastructural examinations, respectively. Quantitative morphometric analysis of myelinated and unmyelinated axonal numbers and diameters and nerve fiber densities may be performed so that disorders of myelin may be identified. Glutaraldehyde fixation also allows for examination of single teased fiber preparations. Evaluation of teased fiber specimens is especially useful for identification of disorders of myelinated fibers. The technique allows for the quantitative assessment of the lengths and morphology of successive myelin internodes in a single nerve fiber. This procedure permits characterization of specific demyelinating processes such as segmental and paranodal demyelination, as well as remyelination.⁴ In addition, information regarding current nerve fiber degeneration can be obtained from examination of teased fiber specimens.

References

1. Braund KG: Nerve and muscle biopsy techniques. Prog Vet Neurol 2: 35, 1980.
2. Braund KG, Walker TL, Vandevelde M: Fascicluar nerve biopsy in the dog. AmJ Vet Res 40: 1025, 1979.
3. Dickinson PJ, LeCouteur RA. Muscle and nerve biopsy. Vet Clin North America Sm Anim Pract 32: 63, 2002.
4. Braund KG. Diagnostic techniques-nerve and muscle biopsy evaluation. In: Braund KG, ed. Clinical syndromes in veterinary neurology. 2nd ed. St. Louis: Mosby, 1994, p 376.